Adaptive non-linear control system

ABSTRACT

An adaptive control system for use with non-linear processes, utilizing a non-linear controller with remotely adjustable deadband width, and an adaptive controller to perform said adjustment. The adaptive controller operates on the same deviation signal as the non-linear controller, but has a discriminator circuit in its input that reacts in opposite directions to high and low-frequency components in the deviation signal. Oscillations of the process at the natural period of the control loop are made to cause the non-linear deadband width to expand, while drift or offset causes it to contract. A 50 to 1 change in process gain can be accommodated with this system. The system is especially successful on non-linear processes such as the pH control of a chemical waste of randomly variable composition. By avoiding continuous cycling, the system can result in reduction in reagent usage of 50 percent, while still meeting effluent specifications.

United States Patent Shinskey ADAPTIVE NON-LINEAR CONTROL SYSTEM [75]Inventor:

Francis G. Shinskey, Foxboro, Mass.

Assignee: The Foxboro Company, Foxboro,

Mass.

Filed: Oct. 31, 1972 Appl. No.: 302,632

ll/l965 Brown et a1. 244/77 9/1972 Green et al. 235/l50.1

Primary Examiner-Eugene G. Botz Attorney, Agent, or FirmNorman E.Brunell Feb. 26, 1974 [57] ABSTRACT An adaptive control system for usewith non-linear processes, utilizing a non-linear controller withremotely adjustable deadband width, and an adaptive controller toperform said adjustment. The adaptive controller operates on the samedleviation signal as the non-linear controller, but has a discriminatorcircuit in its input that reacts in opposite directions to high andlow-frequency components in the deviation signal. Os cillations of theprocess at the natural period of the control loop are made to cause thenon-linear deadband width to expand, while drift or offset causes it tocontract. A 50 to 1 change in process gain can be ac commodated withthis system. The system is especially successful on non-linear processessuch as the pH control of a chemical waste of randomly variablecomposition. By avoiding continuous cycling, the system can result inreduction in reagent usage of 50 percent, while still meeting effluentspecifications.

15 Claims, 7 Drawing; Figures es 70 ADAPTIVE 4 CONTROL DISCRIM. EFUNCTION 28 S DBW \fi ADJ.

52 S PROCESS NoN E CONTROL LINEAR FUNCTION FUNCTION 36 M l I l 34 P D RREAGENT 2o RECORDER CONTROLLED MIXER ELEMENT EFFLUENT our 12 SENSING l4ELEMENT I EFFLUENT IN 69 I PMENTED 3.794. 817 SHEEI1HF4 ADAPTIVE CONTROLDISCRIM. FUNCTION l P A DBW ADJ. v 64% 32 52 PROCESS NON CONTROL LINEARFUNCTION FUNCTION l l 34 P D R REAGENT H/ZO RECORDER CONTROLLED E M'XEREFFLUENT ELEMENT EFFLUENT OUT SENSING ELEMENT FIG.

'l uozumm SHEET 2 BF 4 DEVIATION, E

A KDEAD BAND GAIN zERo DEI-IAD BAND x DBW-- FIG. 20

PH DEVIATION, o

REAGENT FLOW FIG. 2b

PATENIH] FEBZ 61974 SHEET 3 0F 4 PNP 360 f DEVIATION DEADBAND GAIN ADJFIG. 3

PATENTED 3.794.817 v SHEEI s {If 4 {"ElMEvE l POS. HI

I RECTIFIER PASS I :32 so 'r4 NEG, LO-F LO RECT. GAIN PASS FIG. 4

FIG; 50

FIG. 5b

SET POINT ALLOWABLE RANGE l I SET P OINTI ALLOWABLE RANGE BACKGROUND OFTHE INVENTION 1. Field of the Invention This invention relates tosetpoint process control systems of the so-called non-linear" or deadbandcontrol type. In this type of system the gain or proportional band ofthe controller is a function of the magnitude of the deviation betweenthe measured variable and the set point.

A non-linear controller of the type described above finds its greatestapplication in control systems for nonlinear processes where the processgain varies with the magnitude of the deviation. One example of a systemexhibiting this characteristic is a pH controlled process. Incontrolling the pH of the effluent from a chemical plant to comply withthe requirements of the environmental protection laws, the non-linearityof the relationship between the deviation and process gain makes the useof conventional linear forms of control undesirable.

Waste streams from chemical, petroleum, metal, and

The steady-state process gain is the slope of the titration curve at theset point. Increasing deviation of pH from the set point in eitherdirection generally results in lower gain due to the S shape of thecurve. Thus if the gain of a linear pH control loop exceeds unity at theset point, oscillation will expand to the point where the fallingprocess gain reduces the loop gain to unity. Typically, then, pH controlloops tend to limit cycle with an amplitude proportional to the slope ofthe titration curve. Cycling, even within the limits required in aparticular situation, is to be avoided because it wastes expensivereagents.

Because of the non-linear nature of the titration curve, a non-linearcontroller has been devised to provide a more suitable match. Inaddition to the normal three control modes, an additional circuitdevelops a non-linear relationship between the deviation signal,developed by subtracting the measurement value from the set point value,and the error signal acted on by the control modes. This non-linearfunction provides a relatively low gain for values (if deviation withina particular range surrounding the set point. This low gain range iscalled the deadband. Outside of the deadband, a higher gain isavailable. The low gain Within the dead band is intended to minimize thetendency to cycle, while the higher gain regions provide recovery fromupsets to the process. The deadband width is therefore adjusted by theoperator to prevent or reduce limit cy cling while minimizing processdrift.

Although great improvements in the control of nonlinear processes weremade by the recent introduction of non-linear controllers, many problemsstill remain.

Even with a non-linear controller, under certain varia- Although it ispossible to perform similar operations utilizing large digital computersystems, the typical industrial situation is often a small, remotelylocated operation, such as a remote waste treatment facility. Suchfacilities do not justify the large expenditures involved in setting upa digital control system. This invention therefore relates primarily,but not exclusively, to analog control systems as will be describedherein.

2. Description of the Prior Art The art of adaptive control has beendeveloped to correct conceptually related problems in linear controlsystems. The adaptive control systems shown in the prior art generallyfall into tive categories as follows: the model type, the perturbationtype, the limits cycle type, the frequency servo type and the frequencycomparison type. The model type of system uses a simulated model of theprocess to develop a signal indicative of the required gain of thecontroller under similar conditions. When the signal from the modeldiffers from the value of the controller gain an appropriate adjustmentis made.

The perturbation type of system introduces a disturbance and observesthe response of the system due to this perturbation. The system gain isthen adjusted to obtain the desired response characteristics. The limitcycle type of system develops high frequency oscillations (a limitcycle) in the closed loop system. The gain is then adjusted to obtainthe correct limit cycle response. The frequency servo type of systemdetects the low frequency oscillations of the system output around theset point value. The gain of the system is then adjusted to obtain thedesired frequency of oscillation.

The frequency comparison type of system compares the response of thesystem within certain frequency ranges and changes the gain of thesystem to get the desired system response.

A good example of an adaptive controller of this latter type is found inUS. Pat. No. 2,517,081, issued on Aug. 1, 1950 to W. I. Caldwell,entitled Control System with Automatic Response Adjustment. The devicedescribed in that patent is an adaptive pneumatic three-rnode controllerwhose linear proportional gain is linearly increased if the processbegins to drift and decreased if the process begins to oscillate. Inaddition, the controller automatically adapts or adjusts the other twomodes. The rate time and reset time are automatically adjustedproportional to the period of oscillation of the output. These modes arereadjusted only when and if oscillation recurs. This linear controllerwould not be usable with a non-linear process because the non-linearityof the process gain would cause the controller to continuously adaptfrom high to low proportional band gain as the process alternatedbetween con tinuous oscillation and drift.

Adaptive electronic controllers are also known in the art. One exampleis US. Pat. No. 3,535,496, issued on Oct. 20, 1970, to R. M. Bakke whichshows a linear controller whose gain is adapted according to a frequencyanalysis of the loops response to control ac tion. However, thiscontroller would not work effectively with a non-linear process becausethe gain would be continuously adapted as the process gain changed,resulting in limit cycling.

These prior art control systems are not concerned with non-linearcontrollers. In particular, these prior art control systems do not adaptthe deadband of a nonlinear controller in response to changes in processparameters.

SUMMARY OF THE INVENTION It is a significant contribution to the fieldof nonlinear process control to recognize that the abovedescribeddifficulties resulting from the application of a non-linear controllerto a non-linear process are dependent upon the variable relationshipbetween the non-linearity of the process and the non-linearity of thecontroller. In the ideal situation, this relationship would be constantand the resultant loop gain would be independent of deviation.

However, it has been determined that the gains of certain non-linearprocesses, such as a pH controlled effluent, vary not only as a functionof deviation but also as a function of other process variables, inparticular, the composition of the effluent. The high gain band of theprocess therefore effectively varies in width as a function ofunpredictable variables. The deadband of a conventional non-linearcontroller, however, is fixed and cannot continue to cancel the changednonlinearity of the process.

A control system for time varying non-linear processes is thereforeprovided which utilizes a non-linear or deadband electronic controllerto produce a control signal in response to measurement and set pointinputs. In addition, an adaptive controller is utilized to provide awidth adjustment to the non-linear controller to compensate forvariations in the process gain function. The width adjustment is basedon a frequency analysis of the same set point and measurement inputsacted on by the non-linear controller.

The adaptive controller has been designed to adjust the deadband widthso that the resultant loop gain is substantially independent ofdeviation thereby minimizing the tendency to limit cycle.

The heart of the adaptive controller is a discriminator circuit. Thiscircuit separates the deviation into high and low frequency componentsas determined by a crossover-frequency adjustment. The cross-overfrequency is chosen so that the high frequency band includes the naturalperiod of oscillation of the control loop. The two components are thenoppositely polarized and recombined before being sent to theproportional and reset modes of the adaptive controller.

If oscillation in the high-frequency band is present, the discriminatorwill develop a positive signal which will widen the deadband of thenon-linear controller to extinguish the oscillations. If drift or offset(lowfrequency) components exist, the discriminator output will benegative causing the deadband of the non-linear controller to shrink.The adaptive controller is only satisfied with either zero deviation, oroscillation precisely at the cross-over frequency and equallydistributed on both sides of the set point.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagramrepresentation of an adaptive non-linear control system for controllingthe pH of an effluent showing a preferred embodiment of the instantinvention.

FIGS. 2a and 2b are graphical presentations of a nonlinear controllerdeadband and a non-linear process titration curve.

FIG. 3 is a schematic representation of a preferred embodiment of thenon-linear function generator shown in FIG. 1.

FIG. 4 is a block diagram representation of a preferred embodiment ofthe discriminator circuit shown in FIG. 1.

FIGS. 5a and 5b are charts of process measurement signals related to thedescription of burst cycling.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a completeadaptive non-linear control system for the control of the pH of aneffluent as one preferred embodiment of the invention. The effluent tobe measured and controlled enters tank 10 through inlet 12 and isexhausted, either continuously or in batches, through outlet 14. Mixer16 is included with tank 10 to aid in the effluent reagent reaction.Reagent is added to tank 10 through reagent inlet 18 from reagent source20. Reagent flow is controlled by valve 22 which is the controlledelement of the control system to be described below.

Near the outlet side of tank 10 is sensing element 24 which, in the caseof a pH control system, would be a pH sensitive electrode. The signalgenerated by sensing element 24 is amplified by amplifier 25 and may bedisplayed and recorded by recorder 26. This signal serves as themeasured variable input signal M of the control system. Generator 28supplies the set point signal S to the control system.

Subtractor 30 determines the deviation signal E which is the differencebetween measurement and set. This deviation signal is applied to boththe primary control loop (the non-linear controller to be describedbelow) and the adaptive loop. The primary control loop will be describedfirst and then the adaptive loop.

For the purposes of this discussion, an arbitrary distinction will bedrawn between the terms deviation signal E, defined above as thedifference between measurement and set, and error signal E, which is thedeviation signal modified by the non-linear function.

In the primary loop, the deviation signal E is processed by non-linearfunction generator 32 to compensate for the short-term non-linearity ofthe process. The operation of this device and the need for its existencemay be best described with reference to the graphs of FIG. 2.

FIG. 2A is a graph of the error signal E generatedby non-linear functiongenerator 32 in response to deviation E for two values of deadbandwidth. Line A represents E for a deadband width equal to zero. Thiscondition is equivalent to linear control. Line B represents the errorsignal E for a positive value of deadband width DBW. As can be seen fromthe graph, the deadband occurs for values of deviation between +X and X.Between those values, the non-linear function generator has a much lowergain (the deadband gain) than for values of deviation outside of thatrange.

The usefulness of this non-linear function becomes apparent withreference to FIG. 2B. This graph is a titration curve of a pH processshowing pH deviation as a function of reagent flow. The process gain forany point is the derivative of this curve.

As can be seen from the graph, the process has a band of high gain, HGB,for values of deviation between X and X. The process gain outside ofthis band is lower. It can be seen by comparison of FIG. 2A and FIG. 2Bthat if the deadband width DBW of the controller is approximately equalto the high gain band I-IGB of the process, the resultant loop gainwould be nearly constant. This condition results in vastly improvedcontrol as long as the process gain function, i.e., the high gain bandwidth, remains constant.

An electronic circuit capable of generating such a non-linear functionis shown in detail in FIG. 3.

In the operation of this non-linear function generator, the deviationsignal is applied to adjustable voltage divider 38 between inputterminals 36a and 36b. Wiper 39 of this divider is filtered by capacitor48 and serves as the input to 1:1 output amplifier 50. Divider 38 ispartially bypassed by complementary transistors 40 and 42. The bases ofthese transistors are connected to wiper 39 through resistors 44 and 46and also to biasing network 66. If the base voltage of these transistorsequals the emitter voltage, the transistors act like diodes in thecircuit offering high resistance to input signals of less thanapproximately 0.6 of a volt. This creates a i 0.6 volt deadband.

If, however, the base of NPN transistor 40 is biased .6 volt above itsemitter, and the base of PNP transistor 42 is biased .6 volt below itsemitter, the transistors will conduct signals of all voltages andthereby eliminate the deadband. If the transistors are biased in theopposite direction, the DBW increases.

The bias voltages are supplied by biasing network 66 which provides bothmanual and remote DBW adjustment. The manual adjustment is provided bypotentiometer 54 which is connected across a power supply not shown.Wiper 55 of potentiometer 54 is connected to the base of transistor 40.

A pair of zener diodes 56 and 58 are connected across the power supplyin series. The common point between them is tied to DBW ADJ. input 64across which are connected resistors 60 and 62. The common point betweenthese resistors serves as the bias for transistor 42. Adjustment of thevalues of these resistors provides a span adjustment.

It should be noted that the manual DBW ADJ. should be set to zero,except as noted below with regard to burst cycling, when an adaptiveloop is present to provide a remote DBW ADJ. signal. The remote DBW ADJ.signal is always positive so that the manual and remote signals areadditive. Therefore, any DBW ADJ. signal due to potentiometer 54 becomesa minimum DBW when the adaptive loop is present.

Returning now to complete the discussion of the primary loop depicted inFIG. 1, the error signal E, generated by non-linear function generator32, is supplied to control function generator 34 of conventional design.In FIG. 1, the non-linear controller shown consists of set pointgenerator 28, subtractor 30, non-linear function generator 32 andcontrol function generator 34. Except for the non-linear functiongenerator, these components are found in a conventional linearcontroller. The non-linear controller may therefore be convenientlyformed from a three-mode linear controller in which a non-linearfunction generator has been inserted. It is well within the ordinaryskill in the controller art to determine the proper point in a linearcontroller in which to insert the non-linear function generator, and toperform such insertion. It is therefore unnecessary to describe controlfunction generator 34 in any greater detail.

To complete the loop, the control signal output is supplied to valve 22to vary reagent flow from reservoir 20. The system may utilize a singlereagent and a single valve or two reagents and two valves controlled bythe same signal.

The primary control loop described above is of conventional non-lineardesign except for generator 32 which is shown, as required by thisinvention, to have a remotely adjustable DBW. In a conventional system,the DBW would be manually adjusted to a suitable value and allowed toremain at that value.

The instant invention is characterized by the addition, to a non-linearcontrol loop as described above, of the feedback or adaptive loop shownin FIG. 1 which comprises adaptive control function generator 68 andinput discriminator 70. Control function generator 68 is of conventionaldesign and may well be identical to control function generator 34 in aparticular system. In the adaptive loop, however, only a two-modecontroller having proportion and reset modes is required.

In fact, for convenience, discriminator 70, to be described below, maybe fashioned from the unused rate or derivative mode components of theconventional controller whose control function generator was used asadaptive control function generator 68.

A typical discriminator is shown, in block diagram form, in FIG. 4. Thepurpose of this device is to separate and weight the frequency bandspresent in the measurement signal representing oscillation and drift. Toaccomplish this purpose, deviation signal E is processed by high passfilter 72 and low pass filter 74, both of conventional design. Thecutoff or cross-over frequency is chosen so that the high pass filterwill pass a signal oscillating at the natural response frequency of theprocess. This signal is rectified to a positive value by rectifier 76.Any signal that is passed by filter 74, after a gain adjustment isaffected by gain stage 80, is rectified to a negative value by rectifier82. These sig nals are then combined by summer 78, the output of whichis the adaptive error signal 15'.

As shown in FIG. ll, adaptive error signal E" serves as the input toadaptive function generator 68. The control signal output of this unitis supplied to the DBW ADJ. input 64 of non-linear function generator 32to complete the adaptive loop.

In FIG. 1 the adaptive controller shown consists of set point generator28, subtractor 30 (both of which are shared with the non-linearcontroller), discriminator 70 and function generator 68. In an alternateembodiment the adaptive controller may be constructed from a linearcontroller by the addition of a discriminator in the same way that thenon-linear controller may be formed from a linear controller by theaddition of a non-linear function generator. Then, instead of supplyingdeviation signal E directly to the discriminator, it will be necessaryto supply the measurement signal M to the subtractor of the adaptivecontroller and adjust the set point generator to the same value as setpoint generator 28. However, for ease of description, the twocontrollers are shown in FIG. 1 as sharing one subtractor and set pointgenerator.

In operation of this system, the process control function parameters, ofthe non-linear controller, P, D, and R are adjusted in the conventionalmanner. The adaptive control function parameters, P and R, are adjustedso that the integral or reset mode predominates. The last remainingadjustment is the low-frequency gain of the discriminator which is usedto prevent burst cycling. Burst cycling is the repeated bursts ofprocess oscillation that may result from the periodic overnarrowing ofthe deadband in an adaptive system of this type.

In a pH process of the type described it is unlikely that themeasurement would exactly equal the set point for any substantial lengthof time because of the high process gain in the vicinity of the setpoint. In the usual situation the process measurement will driftslightly from the set point even after all major disturbances have diedout. The non-linear controller will attempt to eliminate this drift oroffset utilizing the low gain of the deadband. While this is occuring,however, the adaptive loop will cause the deadband to narrow. Withoutthe low-frequency gain unit the deadband will be narrowed too muchbefore the non-linear controller is able to correct the drift. Even asmall disturbance will then be able to drive the measurement outside ofthe deadband. Limit cycling will occur until the adaptive loop causesthe deadband to widen enough to quench the oscillation. When theoscillation dies out the adaptive loop will narrow the deadband and thelimit cycling will recur. Burst cycling of this type is to be avoidedbecause it wastes expensive reagents and interferes with control. Toprevent burst cycling it is only necessary to keep the deadband frombecoming over-narrow. This may be accomplished by either providing aminimum DBW or by decreasing the rate at which the deadband is narrowedby the adaptive loop.

A minimum DBW may be provided by adjusting potentiometer 54. The remoteDBW ADJ. signal supplied by the adaptive loop will then be added to theminimum DBW determined by this setting. The disadvantage in using thisprocedure to reduce burst cycling is that the minimum DBW reduces therange of the control system and therefore the range of process gainvariation that may be effectively handled.

The preferred method of reducing burst cycling is to utilize lowfrequency gain stage 80. Adjustment of gain stage 80 can vary thelow-frequency gain from unity, i.e., equal to the high frequency gain,to zero. Reducing the low-frequency gain of the discriminator decreasesthe rate at which the adaptive loop narrows the deadband relative to therate at which the deadband is widened. Drift within the deadband maythen be reduced by the non-linear controllerbefore the deadband becomesover-narrowed.

FIG. b is a representation of a chart that was generated by recorder 26in an installation according to the embodiment of the invention shown inFIG. 1 without low-frequency gain stage 80. In FIG. 5b the set pointequals 7.2 pH units and the allowable range is from 6 to 9 pH units.During each of the four major bursts the measurement signal went beyondthe allowable range. These periodic bursts would continue until thesituation was corrected. To prevent the effluent from being dischargedin violation of the laws, a backup system is used to immediately blockoutlet 14 until the measurement returns to the allowable range.

FIG. 5a is a representation of another chart generated in the samemanner with a system that includes low-frequency gain unit 80 adjustedto a gain 0.5 times the gain of the high pass side of the discriminator.This chart shows greater drift within the allowable range and lesshigh-frequency oscillation. High frequency oscillation is to be avoidedbecause it wastes expensive reagent. The high-frequency oscillation thatbegan at approximately 3:30 PM was caused by a change in the compositionof the effluent that entered tank 10 and resulted in a change of processgain. The other oscillations shown in FIG. 5a were due to load changes.

In a typical installation of a non-linear adaptive con trol system asdescribed above, the following approximate parameter settings could beused to advantageously control a Ph process having a natural period of 2minutes.

Primary Loop:

Proportional Band percent Reset Time--l minute Derivative time-0.2minutes Minimum Deadband Gain0.02 (of the proportional gain) DeadbandWidth-i 1.5 pH units (adjustable from 0 to i 3.0 pH units by theadaptive loop) Adaptive Loop:

Proportional Band-300 percent Reset Time-3 minutes Cross-Over Period 1 2minutes Low-Frequency Gain-0.7

The above-described values are merely illustrative and are not to beinterpreted as necessary or limiting to the invention.

While the invention has been described above with reference to apreferred embodiment, it should be understood that it is not intended tolimit the invention to that alternative, but to cover all alternatives,modifications and equivalents as may be included within the spirit andscope of the invention as defined by the appended claims.

I claim:

I. An improved process control system, for use with a process having anon-linear process gain function, of the type normally having:

means for generating a measurement signal related to the value of aprocess variable,

means for generating an error signal non-linearly related to thedeviation of the measurement signal from a preselected set point value,said error signal means having a low gain deadband region for values ofmeasurement signals within a particular range of the set point value anda substantially linear, higher gain region for values of measurementsignals outside of the deadband range, and

control means responsive to the error signal for manipulating theprocess to reduce the magnitude of the error signal, wherein theimprovement comprises:

means for detecting changes in the process gain function; and

means to adapt the non-linearity of the error signal means to vary thewidth of the deadband region in response to changes in the process gainfunction.

2. The improved control system of claim 1 wherein the detecting meanscomprises:

a discriminator responsive to the deviation of the measurement signalfrom a pre-selected reference value for generating an adaptive errorsignal related to changes in the process gain function.

3. The improved control systems of claim 2 wherein the pre-selectedreference value is equal to the set point value.

4. The improved control system of claim 2 wherein the discriminator isresponsive to the frequency characteristics of the measurement signal.

5. The improved control system of claim 4 wherein the discriminatorcomprises:

a plurality of filters for detecting the presence of particular rangesof frequencies in the measurement signal.

6. The improved control system of claim 5 wherein the discriminatorfurther comprises:

a plurality of rectifiers responsive to the filters to generate signalsrepresentative of the frequency ranges present in the measurementsignals; and

means to combine the rectifier signals in a predetermined manner togenerate the adaptive error signal.

7. The improved control system of claim 6 wherein the discriminatorfurther comprises:

a low pass filter responsive to the presence of frequencies below apre-selected cross-over frequency;

a low frequency rectifier for rectifying the output of the low passfilter;

a high pass filter responsive to the presence of frequencies above thecross-over frequency;

a high frequency rectifier for rectifying the output of the high passfilter in an opposite sense to the low frequency rectifier; and

a summer for combining the outputs of the rectifiers to generate theadaptive error signal.

8. The improved control system of claim 7 wherein the discriminatorfurther comprises:

a gain stage for changing the gain of one filter rectifier combinationwith respect to the other to pre vent burst cycling.

9. The improved control system of claim 1 further comprising:

means to provide a minimum deadband width to prevent burst cycling.

10. The improved control system of claim 1 wherein the detecting meanscomprises:

a discriminator responsive to the presence of particular ranges offrequencies in the measurement signal for generating an adaptive errorsignal related to the dominant frequency range.

11. The improved control system of claim 10 wherein the adaptive meanscomprises:

means for increasing the deadband width in response to an adaptive errorsignal characterized by the predominance of a frequency range above apreselected cross-over frequency; and

means for decreasing the deadband width in response to an adaptive errorsignal characterized by the predominance of a frequency range below thecross-over frequency.

12. The improved control system of claim 11 wherein the pre-selectedcross-over frequency is below the natural frequency of the process.

13. A control system for use with non-linear process comprising:

a non-linear primary control loop for operating on the process toeliminate any deviation between a measured process variable and apreselected set point;

means responsive to the deviation to narrow th deadband when the processdrifts; and

means responsive to the deviation to widen the deadband when the processoscillates.

14. The control system of claim 13 further comprising:

means to provide a minimum deadband to prevent burst cycling.

15. The control system of claim 13 further comprismeans to decrease therate at which the deadband is narrowed.

1. An improved process control system, for use with a process having anon-linear process gain function, of the type normally having: means forgenerating a measurement signal related to the value of a processvariable, means for generating an error signal non-linearly related tothe deviation of the measurement signal from a preselected set pointvalue, said error signal means having a low gain deadband region forvalues of measurement signals within a particular range of the set pointvalue and a substantially linear, higher gain region for values ofmeasurement signals outside of the deadband range, and control meansresponsive to the error signal for manipulating the process to reducethe magnitude of the error signal, wherein the improvement comprises:means for detecting changes in the process gain function; and means toadapt the non-linearity of the error signal means to vary the width ofthe deadband region in response to changes in the process gain function.2. The improved control system of claim 1 wherein the dEtecting meanscomprises: a discriminator responsive to the deviation of themeasurement signal from a pre-selected reference value for generating anadaptive error signal related to changes in the process gain function.3. The improved control systems of claim 2 wherein the pre-selectedreference value is equal to the set point value.
 4. The improved controlsystem of claim 2 wherein the discriminator is responsive to thefrequency characteristics of the measurement signal.
 5. The improvedcontrol system of claim 4 wherein the discriminator comprises: aplurality of filters for detecting the presence of particular ranges offrequencies in the measurement signal.
 6. The improved control system ofclaim 5 wherein the discriminator further comprises: a plurality ofrectifiers responsive to the filters to generate signals representativeof the frequency ranges present in the measurement signals; and means tocombine the rectifier signals in a predetermined manner to generate theadaptive error signal.
 7. The improved control system of claim 6 whereinthe discriminator further comprises: a low pass filter responsive to thepresence of frequencies below a pre-selected cross-over frequency; a lowfrequency rectifier for rectifying the output of the low pass filter; ahigh pass filter responsive to the presence of frequencies above thecross-over frequency; a high frequency rectifier for rectifying theoutput of the high pass filter in an opposite sense to the low frequencyrectifier; and a summer for combining the outputs of the rectifiers togenerate the adaptive error signal.
 8. The improved control system ofclaim 7 wherein the discriminator further comprises: a gain stage forchanging the gain of one filter rectifier combination with respect tothe other to prevent burst cycling.
 9. The improved control system ofclaim 1 further comprising: means to provide a minimum deadband width toprevent burst cycling.
 10. The improved control system of claim 1wherein the detecting means comprises: a discriminator responsive to thepresence of particular ranges of frequencies in the measurement signalfor generating an adaptive error signal related to the dominantfrequency range.
 11. The improved control system of claim 10 wherein theadaptive means comprises: means for increasing the deadband width inresponse to an adaptive error signal characterized by the predominanceof a frequency range above a pre-selected cross-over frequency; andmeans for decreasing the deadband width in response to an adaptive errorsignal characterized by the predominance of a frequency range below thecross-over frequency.
 12. The improved control system of claim 11wherein the pre-selected cross-over frequency is below the naturalfrequency of the process.
 13. A control system for use with non-linearprocess comprising: a non-linear primary control loop for operating onthe process to eliminate any deviation between a measured processvariable and a preselected set point; means responsive to the deviationto narrow the deadband when the process drifts; and means responsive tothe deviation to widen the deadband when the process oscillates.
 14. Thecontrol system of claim 13 further comprising: means to provide aminimum deadband to prevent burst cycling.
 15. The control system ofclaim 13 further comprising: means to decrease the rate at which thedeadband is narrowed.